1. Field of the Invention
This invention relates to a process for molding articles.
2. Description of the Prior Art
It is desirable to be able to fabricate, by a low cost process, sheet material having closely spaced narrow ribs extending thereacross wherein the sheets have uniform properties throughout and perhaps a higher density in the rib portion. In the molding of such an article problems result from the fact that it is difficult to uniformly fill and compact a powder mixture within the rib forming grooves of the mold. This is even more of a problem when the molding material contains fibers since the material is not free flowing because the fibers tend to log jam as the material is being sifted and spread into the narrow grooves. Hidden voids can occur within the grooves, and these voids do not necessarily disappear when the material is compacted such as with a flat plate punch or with a roller. A punch or roller does not pack the material into the grooves with adequate density and uniformity; and often the result is a higher density in the web portion of the finished article as compared to the rib portion. All of these problems become accentuated as the fiber content of the molding material increases.
One application for ribbed sheet material is as a fuel cell electrode substrate wherein a suitable catalyst is disposed on the non-ribbed surface of the sheet. Electrodes comprising ribbed substrates are described in commonly owned U.S. Pat. No. 4,115,627. One method for making ribbed fuel cell substrates is described in commonly owned U.S. Patent Application Ser. No. 824,758 titled "Method for Fabricating a Ribbed Electrode Substrate" by G. Sandelli filed on Aug. 15, 1977. In the one specific example set forth in that application the molding material comprises 20% resin and 80% carbon fibers. This mixture is sifted through a screen into a molding die having the mirror image of the rib pattern desired in the substrate. The mixture is pressed to a preset thickness by a flat plate punch and is then heat treated. The finished part had a porosity of approximately 90% in the ribs and 65% in the connecting web portions despite it being preferable to have more uniform characteristics throughout the substrate.